Carol Hemingway was the fifth of six children born to Clarence and Grace Hemingway. She was born two years before Ernest's twelfth birthday prompting Ernest to once call her his "own special present." Their relationship was very good in Carol's early years, but deteriorated when Carol decided to marry John Gardner. Ernest, who was protective of his sister and became her legal guardian after the death of their father in 1928, forbade the union and stated that it would never last.
Carol Hemingway and John Gardner married on June 25, 1933 and had three children together: Elizabeth, Hilary, and Mark. Their marriage lasted for sixty-five years. In a 2002 interview, Carol's daughter Elizabeth gave a possible reason why Ernest Hemingway disapproved so much of her father: "My father, who practically never mentioned my mother's brother, was asked in his last year what he had said to make Ernest so angry. He responded, 'I believe I called him an overgrown boy scout.' In later years, my dad was mortified by how cocky he had been as a college student."
On October 27, 2002, Carol Hemingway died at the age of ninety-one. She was the last living sibling of Ernest Hemingway.